Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include portable computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code, system data such as a basic input/output system (BIOS), and other firmware can typically be stored in flash memory devices. Most electronic devices are designed with a single flash memory device.
Flash memory devices typically have controller circuits that are responsible for executing operations of the memory device. For example, the controller may execute instructions for erasing or programming the memory cells. An increase in the complexity of the flash memory devices has created a need for more complex controller circuits on the device.
Some flash memory devices use a microsequencer as the controller circuit. This is an internal state machine that steps through a sequence of operations to achieve a certain goal. The microsequencers rely on hard coded sequences for the different operations of the flash memory device.
Typical flash memory uses a single bit-per-cell. Each cell is characterized by a specific threshold voltage or Vt level. Electrical charge is stored on the floating gate of each cell. Within each cell, two possible voltage levels exist. These two levels are controlled by the amount of charge that is programmed or stored on the floating gate; if the amount of charge on the floating gate is above a certain reference level, the cell is considered to be in a different state (e.g., programmed, erased).
Multilevel cells have recently been introduced to greatly increase the density of a flash memory device. This technology enables storage of multiple bits per memory cell by charging the floating gate of the transistor to different levels. This technology takes advantage of the analog nature of a traditional flash cell by assigning a bit pattern to a specific voltage range. This technology permits the storage of two or more bits per cell, depending on the quantity of voltage ranges stored on the cell.
With the introduction of multilevel cells, the precision at which the voltages on the device are sensed must greatly increase. This increases the complexity of testing the memory devices. For example, the analog voltages used on the devices must be set on a die-by-die basis due to minute differences in the die composition. This greatly increases test time and test cost.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a more flexible method of testing memory devices.